Delirium (or ‘acute confusional state’) is a severe neuropsychiatric disorder characterised by acute and fluctuating deficits in attention, arousal and cognitive function. Despite the prognostic importance of early diagnosis and treatment of delirium, it is currently under-studied and is under-detected in clinical settings.
‘Inattention’ is recognised as being one of the core features of delirium. However, the particular aspects of attentional disturbance associated with this feature have not been well characterised. Rather, these attentional deficits have more typically been described in patients using subjective clinical expressions, such as ‘being easily distractible’ or ‘having difficulty paying attention’. This is important because a better understanding of the specific neuropsychological processes that are impaired in delirium may provide key insights to understanding the pathophysiological mechanisms that underlie it. Clinical tests that can reliably detect attentional deficits may be particularly useful for providing diagnostic measures that are sensitive to delirium. However, there is overlap between the neuropsychological features of delirium and those of other conditions, for example dementia, and many known tests that can detect attentional deficits cannot distinguish reliably between delirium and such other conditions. This may be because (a) the tests are too challenging, such that even the milder attentional deficits observed in mild to moderate Alzheimer's dementia are enough to result in reduced scores, and (b) because the category or type of attentional deficits are not specific to delirium.
The Cognitive Test for Delirium (CTD), as described by Hart et. al. in “Abreviated Cognitive Test for Delirium”, Journal of Psychosomatic Research, Vol. 43, No. 4, pages 417-423 (1997) and also in “Validation of a Cognitive Test for Delirium in Medical ICU Patients” in Psychosomatics, Volume 37, Number 6, page 533 (1996), includes two tasks that require patients to listen to strings of serially-presented letters, and to make a response each time a given target letter is heard. These tasks therefore depend on the ability to sustain attention to auditory information over an extended period of time. Intensive care patients with delirium show impairments on these tasks, indicating the presence of sustained attentional deficits, and the test may be able to provide some distinction between patients with delirium and patients with depression, dementia or schizophrenia. However, the tests have a significant subjective element as they are performed by a human tester, and the outcome of the tests may depend on the way in which the tests are performed by the tester. Moreover, given the auditory nature of these tasks, it is unclear how well these tasks would transfer into noisier, general ward settings. In addition, the patient's performance on the task does not tell us whether they also have deficits in attending to information from non-auditory modalities, such as vision or touch.
Lowery et al, in “Quantifying the association between computerised measures of attention and confusion assessment method defined delirium: a prospective study of older orthopaedic surgical patients, free of dementia”, Int J Geriatr Psychiatry (2008), DOI: 10.1002/gps.2059, showed that patients with delirium also perform worse than cognitively-healthy control patients on two computerised tasks that involve sustained attention to visual information. In these tasks, patients were required to attend to series of visual stimuli, and to make speeded button responses to particular target stimuli. The information was presented on a high resolution computer screen, and the responses recorded via a module containing two buttons, one marked ‘NO’ and the other ‘YES’. The test took approximately 5 minutes to perform, and included the Digit Vigilance (DV) and Choice Reaction Time (CRT) tasks. For the DV task, the participants were required to use the module to identify multiple presentations of a ‘target’ digit within a two minute period of serially presented ‘target’ and ‘distracter’ digits. The CRT task required the participant via the module to press either ‘NO’ or ‘YES’ as they appeared on the screen. Twenty presentations of either stimulus were displayed sequentially with a varying inter-stimulus interval. The DV task provided a measure of ability to sustain attention through accuracy of response. The CRT task provided two measures, Mean reaction time (msec) and intra-trial variability of reaction time (standard deviation of the reaction times across 90 sec for each individual participant).
However, as these tasks require relatively rapid perceptual processing of visual information, and also the speeded formulation and execution of motor responses, they are not specific to detecting attentional deficits, that is, they depend on adequate functioning of several cognitive domains other than sustained attention, indeed, patients with Alzheimer's dementia are known to perform poorly on these tasks, thereby limiting their specificity to detecting delirium. Furthermore, as patients with delirium have also been shown to have deficits in visual perceptual processing, it is possible that some of their difficulties on these tasks may in fact reflect the perceptual demands of the tasks rather than the attentional components.